Resistor and resistor composition



Sept. 1962 M. E. DUMESNIL 3,052,573

RESISTOR AND RESISTOR COMPOSITION Filed March 2, 1960 2- Resistor unit 1- Ceramic bose .FIG. 2

5- Metol oxide coating and particles 4-Gloss matrix 0 pflrficles l-Ceromic base INVENTOR MAURICE E. DUMESNIL ATTORNEY United States Patent 3,052,573 RESISTOR AND RESISTOR COMPOSITION Maurice Edward Dumesnil, Metuchen, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Mar. 2, 1960, Ser. No. 12,256 13 Claims. (Cl. 117-221) This invention relates to ceramic electrical resistor compositions and to electrical resistor films produced therewith. More particularly, the invention relates to vitreous enamel-palladium oxide compositions which may be applied to and fired on a ceramic dielectric material to produce an electrical resistor, and electrical resistors so produced.

The recently issued patent to J. B. DAndrea, No. 2,924,540, discloses a new type of ceramic resistor composition and electrical resistors produced therefrom, comprising finely divided palladium metal, with or without addition of silver, in combination with a vitreous enamel flux. Although the DAndrea type resistor material constituted a great improvement over previously known types of printed ceramic resistor material, it was not altogether satisfactory since the use of different vitreous enamel frits of different maturing temperatures gave resistors of rather widely divergent resistances.

It is an object of this invention to produce vitreous enamel containing resistor compositions that will have substantially uniform resistances even when mixed with vitreous enamels of varying maturing temperatures and fired at such varying temperatures.

It is another object of this invention to produce vitreous enamel containing resistor compositions of high stability to firing temperatures.

It is yet another object to produce a more easily reproducible electrical resistor.

Other objects of the invention will appear hereinafter.

The resistor compositions of this invention may be prepared by mixing finely divided palladium oxide Wi-th finely divided vitreous enamel frit. The dry finely divided palladium oxide-enamel frit composition may, if desired, be

-miXed with a liquid or pasty vehicle for application to a ceramic dielectric.

The palladium oxide may, if desired, be replaced by rhodium oxide with substantially like results. Since RhO has substantially the same characteristics as PDO for the purposes and objects of this invention, where reference is made to Pd or PdO throughout the specification and claims it is to be understood that the Pd or PdO may be respectively replaced by Rh or RhO.

As in the DAndrea patent, so that the voltage and temperature coefficients of the resulting resistors may be more readily controlled, it is usually preferred that a quantity of finely divided (less than 325 mesh) silver, gold or platinum be added to the palladium oxide in the resistor composition.

The finished resistor may be produced by applying the resistor composition to a ceramic dielectric and firing the same at a temperature that will fuse (mature) the vitreous enamel to the dielectric surface.

A still further improvement in reproducibility of resistors of a given resistance value maybe obtained by the use of finely divided palladium oxide with an oxide of a metal from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn, and the rare earth metals bonded to it.

Finely divided palladium oxide for use in accordance with this invention may be prepared by heating finely divided palladium metal in air. The palladium metal may be produced by chemical precipitation or by mechanical comminution or disintegration in known manner. It is highly desirable, however, that the fineness of division be less than 325 mesh and preferably of an average particle size of between 0.1 and 50 microns. When palladium metal powder is heated in air it begins to oxidize readily at 300 C. and reaches a maximum Weight gain at 700 C. to 750 C. to form the non-stoichiometric structure PdO (1), being essentially palladium monoxide. Beyond this temperature the oxide becomes unstable and reverts to the metal in the same way that silver oxide decomposes to metal at about 300 C. The use of PdO in place of metallic Pd in vitreous resistor compositions oifers the advantage of using very soft vitreous enamel (450600 C.) as well as vitreous enamel having a fairly high melting point (700800 C.) without completely varying the resistance values of the finished resistors produced therefrom.

The finely divided palladium oxide with or without silver, gold or platinum powder, is mixed with the finely divided (less than 325 mesh) vitreous enamel flux in the following proportions:

[Percentages by weight] Operable Preferred proportions proportions 4 to 4 to 15 0 to 50 0 to 12 8 to 90 8 to 27 92 to 10 92 to 73 firing period and a 10 fold variance with a varying firing period of 30 to 90 minutes under a constant firing temperature.

It is therefore greatly to be preferred to prepare the resistor compositions with PdO modified by the presence of certain other metal oxides. For examples, finely divided PdO containing 10% by weight of Zr0 will produce a resistor the resistance of which will vary by only 20% when fired during a given period at a temperature varying between 1300 F. and 1500 F., and will vary by about 2 fold when fired at periods varying between 30 and 90 minutes at a constant temperature between 1300" F. and 1400 F. and less than 3 fold at similar varying periods at 1500 F. The similar addition of the other listed metal oxides to PdO will produce improvements of the same order.

Where a palladium oxide is desired that is associated with a metal oxide of the kind referred to above, it may be obtained in any manner whereby at least a portion of the surface of the PdO particles is coated with an ad herent coating of the metal oxide additive. The following proportions of the metal oxides to PdO may be used:

[Percentages by weight] Operable Preferred proportions proportions PdO (replaceable with RhO) 65 to 99. 9 85 to 92 Metal oxide 01' the group Zr, Al, Sr, Ca, Sn,

Mg, Zn, and rare earth metals 35 to 0.1 15 t 8 Resistor formulations prepared with PdO modified with one of said metal oxides, silver and Vitreous enamel frit may contain the following proportions:

[Percentages by weight] As above stated, the additive metal oxide may be the oxide of Zr, Al, Sr, Ca, Sn, Mg, Zn or a rare earth or combinations thereof. The rare earths consist of the metals scandium, yttrium, lanthanum, cerium prascodymium, neodymium, Samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, neoytterbium, and lutecium. Of these the most common are probably lanthanum and cerium, both of which have given good results as PdO additives in accordance with this invention. The characteristics of the rare earths are very much alike and although they would undoubtedly all be operative for the purposes of this invention, many of them are probably too rare and expensive to be practical.

Resistors prepared in accordance with this invention are illustrated in the accompanying drawings, in which,

FIGURE 1 is a cross-sectional view, on a highly exaggerated scale, of a resistor produced in accordance with this invention.

FIGURE 2 is a smaller cross-sectional view of a second embodiment of the resistor of this invention.

Referring to the drawings, 1 designates a ceramic dielectric base such as a glass, porcelain, refractory, barium titanate or a similar dielectric base. A resistor unit 2 is formed on the base 1 by firing a resistor composition of the invention and comprising PdO or RhO particles mixed with a glass frit. The resulting resistor unit 2 is comprised of the PdO or RhO particles 3 bonded in a glass matrix 4.

FIGURE 2 shows a similar resistor comprised of a ceramic base 1 and containing metal oxide particles 5 both coated on at least parts of the surfaces of the PdO particles 3 and also interspersed bet-ween the PdO particles, all in a glass matrix 4. The metal oxide particles 5 as stated in the specification may consist of Zr, Al, Sr, Ca, Sn, Mg, Zn or a rare earth or combinations thereof.

The following methods are given as suitable methods for preparing the finely divided PdO (replaceable wholly or partially with RhO) containing the additive metal oxide. These methods are not, however, to be taken as limitative of the invention, other methods being equally applicable to the production of the metal oxide modified PdO. r

Preferably, the additive metal oxide is produced from a Water-soluble salt of the metal by decomposition to the oxide on the surface of the PdO particles. This may be accomplished, for example, by dissolving the nitrate salt of the desired metal oxide in water and mixing therewith finely divided palladium metal (less than 325 mesh). The water is then evaporated from the mixture and when dry the residue is placed in a furnace and fired at 750 C. for 1 /2 to 20 hours. The resulting homogeneous mixture of PdO and the additive metal oxide is crushed and ballmilled in the presence of an inert liquid for a period of 2 to 20 hours.

Another method consists in adding the nitrate solution of the additive salt to palladium oxide powder and heat treating as above described.

A third method that is particularly applicable with aluminum nitrate solution comprises the addition of an1- monium hydroxide to palladium chloride solution to form a fine suspension of ammonium chloropalladate to which is then added a measured amount of colloidal hydrous aluminum oxide (formed by adding ammonium hydroxide to aluminum nitrate solution). The mixture is thoroughly stirred and filtered. The residue from the filter is heated at about 750 C. for minutes to produce a finely divided Al O -PdO powder.

The vitreous enamel frit used in the production of the vitreous enamel resistor compositions of the invention may be composed of any glass frit, such as a borosilicate frit, lead borosilicate frit, cadmium, barium, calcium, or other borosilicate frit. The preparation of such frits is well known and consists, for example, in melting together boric oxide, silicon dioxide and lead oxide, cadmium oxide, or barium oxide and pouring such molten composition into Water to form the frit. The batch ingredients may, of course, be any compound that will yield the desired oxides under the fusing conditions of frit production, i.e., boric oxide will be obtained from boric acid or borax, silicon dioxide will be produced from flint, lead oxide will be produced from red lead or white lead, barium oxide will be produced from barium carbonate, etc.

The coarse frit is preferably milled for 2 to 20 hours, for example, in a ball-mill with water. The frit may contain varying amounts of other oxides, such as zinc oxide, magnesium oxide, or the like.

The vitreous enamel frit and palladium oxide powder, with or Without silver, gold or platinum powder, may be mixed in any manner, for example, in a ball-mill, and the resulting dry composition sold as such. The dry composition is, however, preferably first mixed with a liquid vehicle and sold in the form of a liquid or paste. The vehicle may vary widely in composition. Any inert liquid may be employed for this purpose, for example, Water, organic solvents, with or without thickening agents, stabilizling agents, or the like, for example, methyl, ethyl, butyl, propyl or higher alcohols, the corresponding esters such as the acetates, propionates, etc., the terpenes and liquid resins, for example, pine oil, alpha-terpineol, betaterpineol, and the like, and other liquids without limitation, the function of the liquid vehicle being mainly to form a liquid or paste of the desired consistency for application purposes. The vehicles may contain or be composed of volatile liquids to promote fast setting after application, or they may contain waxes, thermoplastic resins, or wax-like materials Which are thermofluid by nature whereby the composition may be applied to a ceramic insulator while at an elevated temperature to set immediately upon contact with the ceramic base.

Any desired amount of vehicle or organic binder may be added whereby to stencil, spray or brush the finely divided metal oxide onto a ceramic dielectric base material. For stencil screen application the concentration of solids and the viscosity of the paste must be controlled since these will affect the thickness of the prints and the resistance value of the resistors. The viscosity of the compositions (preferably about to thousand centipoises) may be adjusted so that the fired fihn is of the desired thickness. A very suitable thickness would be of the order of 15 to 25 microns.

The ceramic dielectric base material may be comprised of any ceramic material that can withstand the firing temperature of the vitreous enamel-palladium oxide composition. For example, glass, porcelain, refractory, barium titanate, or the like may be used. Preferably, the ceramic insulating materials should have a smooth, sub- .5 stantially uniform surface structure but this is not absolutely necessary.

The resistor composition is then applied in a uniform thickness on the ceramic dielectric. This may be done by any of the application methods above disclosed. The dielectric and applied resistor composition is then dried, if necessary, to remove solvent from the vehicle and then fired in a conventional lehr or furnace at a temperature at which the enamel frit is molten, whereby the conductive material is bonded to the ceramic dielectric.

The following examples are given to illustrate certain preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto. In all of the examples the particle size of the palladium, palladium oxide, metal oxides, silver, gold, platinum, and frit averaged between about 1 and 5 microns. Although it is desirable to maintain the particle sizes fairly consistent to obtain good reproducible results, the actual particle sizes used are not critical.

Example I 10.44 grams of zirconyl nitrate are dissolved in water in a large evaporating dish. To this is added 23.47 grams of finely divided palladium metal. The resulting mixture is heated with continuous stirring until all the water is evaporated therefrom, after which the dry mixture is placed in a furnace and fired for 90 minutes at a temperature of 750 C. The fired product, after cooling, is then ball-milled in acetone for a period of 16 hours. The resulting finely divided metal oxide contained about 90% PdO and ZrO Example 11 44 grams of aluminum nitrate is dissolved in water and 30 grams of finely divided palladium powder is added thereto. The mixture is dried, fired and ball-milled in the manner described in Example I. If a still finer particle size is desired, the ball-milled product may be further milled with glass beads for 16 to 70 hours. The dried ball-milled product contained about 85% PdO and A1203- Example III A resistor composition was prepared by blending: 60 grams of the ball-milled product of Example I, 40 grams of commercial precipitated silver having a particle size of about 2 microns, 180 grams of lead borosilicate frit consisting of 65% PbO, 10% B 0 and 25% SiO and 130 grams of a vehicle composed of beta-terpineol viscosified with 6% by weight of ethyl cellulose. The ethyl cellulose used was that sold under the designation T-200 lay Hercules Powder Company of Wilmington, Delaware.

The resulting composition was roll-milled thoroughly and then printed with a thickness of about 1 mil. with a 200 mesh stainless steel squeegee stencil screen onto a barium titanate dielectric disc and fired to produce electric resistors. The following table gives the resistance per square in K 1000 ohms) of the resulting resistors obtained using given firing times and temperatures.

Similar results were obtained by using alumina or steatite dielectric discs in place of the barium titanate dielectric.

Example IV A paste resistor composition was prepared by the procedure of Example III with the sole exception that the 16-hour ball-milled product of Example H replaced the product of Example I.

v minutes Firing time 1,300 F. 1,400" F. l,500 F.

30 minutes 2. 1 3. 7

Example V A resistor composition was prepared by the procedure of Example IV but containing the metal oxide product of Example II ball-milled 16 hours in acetone and beadmilled further for 16 more hours. A resistance per square ranging from 10K to 15K was obtained using the firing conditions of Example IV.

Example VI Following the procedure of Example III using the metal oxide product of Example II, the glass frit of Example III, finely divided silver powder having a particle size of about 2 microns, in the proportions by weight given in the following table and firing the printed resistors at 1400 F. for 60 minutes, the resistance Values set forth in the table were obtained:

Pd0(85), A1zO (l5)OfEXaD1- 20 20 30 25 20 25 25 10 ple II, percent Silver powder, percent 1O 15 7.5 15 17.5 20 25 30 Glass frit of Example III,

percent 70 62.5 60 62.5 55 50 60 Resistance per square (taken to nearest whole numbers),

Example VII Following the procedure of Example III but replacing the metal oxide product of Example I with the following metal oxide combinations, produced substantially in accordance with the procedure of Example I, the following resistance values were obtained at the various given times and temperatures of firing:

Example VIII Two resistor compositions were prepared by the procedure of Example III using as the glass frit a lead borate glass having a softening point of 430 C., a finely divided precipitated silver powder having a particle size of about 2 microns, and as the metal oxide, one of the compositions is formulated with the aluminum oxide modified PdO of Example II and the other composition with PdO prepared by heat treating finely divided Pd in normal atmosphere at 750 C. for 16 hours and ball-milled for 16 hours. The two compositions contained the following compositions of total metal oxide, silver powder and lead borate frit.

The compositions were placed on barium titanate alumina and steatite dielectric discs by the procedure of Example III. The resistance values of the resulting resistors fired at the given times and temperatures were as follows:

Throughout the examples where silver is used, it can be replaced wholly or partially with either gold or platinum or both with substantially similar results, and where PdO is used it can be replaced wholly or in part by RhO.

The resistor compositions of this invention are outstandingly useful for the production of electrical resistors having good stability, low temperature coeflicient of resistance (the percentage change in resistance at two given temperatures divided by the ditference in temperature between said two temperatures in degrees centigrade) low voltage coefficient of resistance (the percentage chang in resistance at two given voltages divided by the difference between said two voltages) being readily reproducible and being relatively non-sensitive to times and temperatures of firing.

A resistor composition formulated with PdO and a vitreous enamel (glass) frit will produce resistors having a positive temperature coefficient of resistance (TCR) varying between 0.05% and 0.08% per degree C. On the other hand, a resistor composition formulated with PdO modified with another of the aforesaid metal oxides will produce resistors with either a positive or negative TCR. This TCR is dependent on the type of the added oxide and its relative proportion to the PdO, on the heat treatment and fineness of division of the metal oxide combination, the resistor formulation (i.e., ratio of metal oxide, silver and glass frit) and the type of dielectric on which the resistor composition is printed. It has been found that by suitable control of these variables the TCR does not vary widely with the firing conditions of the resistor. In this way a uniform and reproducible TCR of less than 0.01% per degree C. may be maintained, if desired.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A resistor composition adapted to be applied to and fired on a ceramic dielectric to form readily reproducible electric resistors consisting essentially of 4% to 90% finely divided metal oxide taken from the group consisting of PdO and RhO, as a conductive component, to 50% finely divided metal taken from the group consisting of Ag, Au and Pt as a conductive component, a total of 8% to 90% of said conductive component and 10% to 92% finely divided vitreous enamel frit.

2. A vitreous enamel resistor composition as defined in claim 1 in combination with sufficient inert liquid vehicle to form a paste with said composition components.

3. A resistor composition adapted to be applied to and fired on a ceramic dielectric to form readily reproducible electric resistors consisting essentially of 4% tofinely divided metal oxide taken from the group consisting of PdO and RM), as a conductive component, 0% to 12% finely divided metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 8% to 27% of said conductive component and 73% to 92% finely divided vitreous enamel frit.

4. A vitreous enamel resistor composition as defined in claim 3 in combination with suflicient inert liquid vehicle to form a paste with said composition components.

5. A resistor composition adapted to be applied to and fired on a ceramic dielectric to form readily reproducible electric resistors consisting essentially of 10% to 70% finely divided metal oxide taken from the group consisting of PdO and RhO containing bonded thereto 0.1% to 35 by weight of a metal oxide taken from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn and the rare earth metals, as a conductive component, 0% to 30% finely divided metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 10% to 70% of said conductive component, and 30% to finely divided vitreous enamel frit.

6. A vitreous enamel resistor composition as defined in claim 5 in combination with sufiicient inert liquid vehicle to form a paste with said composition components.

7. A resistor composition adapted to be applied to and fired on a ceramic dielectric to form readily reproducible electric resistors consisting essentially of 10% to 70% finely divided metal oxide taken from the group consisting of PdO and R containing bonded thereto 8% to 15 by weight of a metal oxide taken from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn and the rare earth metals, as a conductive component, 0% to 30% finely divided metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 10% to 70% of said conductive component and 30% to 90% finely divided vitreous enamel frit.

8. A vitreous enamel resistor composition as defined in claim 7 in combination with sufficient inert liquid ehicle to form a paste with said composition components.

9. A resistor composition adapted to be applied to and fired on a ceramic dielectric to form readily reproducible electric resistors consisting essentially of 15% to 35% of finely divided metal oxide taken from the group consisting of PdO and RhO containing bonded thereto 0.1% to 35% by weight of a metal oxide taken from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn, and the rare earth metals, as a conductive component, 5% to 25% finely divided metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 20% to 60% of said conductive component, and 40% to 80% finely divided vitreous enamel frit.

10. A vitreous enamel resistor composition as defined in claim 9 in combination with sufficient inert liquid ve hicle to form a paste with said composition components.

11. An electrical resistor comprising a ceramic dielectric containing on the surface thereof a vitreous enamel resistor element comprising between 10% and 70% of a metal oxide taken from the group consisting of PdO and RhO containing bonded thereto 0.1% to 35 by weight of a metal oxide taken from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn and the rare earth metals, as a conductive component, 0% to 30% metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 10% to 70% of said conductive component, said conductive component embedded in a glass matrix.

12. An electrical resistor comprising a ceramic dielectric containing on the surface thereof, a vitreous enamel resistor element comprising between 15 and 35 of a metal oxide taken from the group consisting of PdO and F110 containing bonded thereto 0.1% to 35 by weight of a metal oxide taken from the group consisting of Zr, Al, Sr, Ca, Sn, Mg, Zn, and the rare earth metals, as a conductive component, 5% to 25% metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 20% to 60% of said conductive component, said conductive component embedded in a glass matrix.

13. An electrical resistor comprising a ceramic dielectric containing on the surface thereof a vitreous enamel resistor element consisting essentially of 4% to 90% of a 5 finely divided metal oxide taken from the group consisting of PdO and RhO, as a conductive component, 0% to 50% finely divided metal taken from the group consisting of Ag, Au and Pt, as a conductive component, a total of 8% to 90% of said conductive component, said 0011- 10 ductive components embedded in a glass matrix.

References Cited in the file of this patent UNITED STATES PATENTS Iira May 4,

Christensen et a1. Feb. 15,

Huttar June 3,

DAndrea Feb. 9,

Place et al. Aug. 30,

FOREIGN PATENTS Great Britain June 8, 

1. A RESISTOR COMPOSITION ADAPTED TO BE APPLIED TO AND FIRED ON A CERAMIC DIELECTRIC TO FORM READILY REPRODUCIBLE ELECTRIC RESISTORS CONSISTING ESSENTIALLY OF 4% TO 90% FINELY DIVIDED METAL OXIDE TAKEN FROM THE GROUP CONSISTING OF PDO AND RHO, AS A CONDUCTIVE COMPONENT, 0% TO 50% FINELY DIVIDED METAL TAKEN FROM THE GROUP CONSISTING 